The invention relates to an electrochemical storage device of the type having a plurality of serially connected cells, in which each has two spacially separated electrodes.
German patent document DE 42 29 437 C1 discloses an electrochemical storage device of this generic type, in which a storage device housing surrounding the cells has terminals of differing polarity extending therefrom. Because of the series connection of the individual cells, the maximum voltage of such a storage device is calculated as the number of individual cells multiplied by their specific voltage. Thus, these electrochemical storage devices can have a maximum voltage of a few hundreds of volts, and are, therefore, of particular interest for traction batteries for motor vehicles.
Since the components of the individual cells (electrodes, electrolyte, weight of the active masses, distribution etc.) cannot be manufactured to be completely identical, the individual cells during charging attain their maximum charge state at different times. If those cells which are already completely charged continue to be subjected to a charge voltage that exceeds the decomposition voltage of the individual components of the individual cells, they may be damaged, or even destroyed. In the case of electrochemical storage devices having a bipolar sandwich construction (positive electrode, bipolar plate, negative electrode, electrolyte, positive electrode etc.), this risk is considerable, since the current flows transversely through the thin functional plates or sheets placed next to one another in layers, and the electrodes are not equipped with current collector lugs between which a protective element can be interposed. Furthermore, appropriate contacting would at least be very complicated, since the thickness of the individual cell elements may be less than 1 mm. In some cases, the electrolytes and/or the electrodes are even fashioned as a thin sheet.
German patent document DE 28 19 584 C2 discloses a protective circuit for a plurality of electrochemical storage devices connected in series, or sets of electrochemical storage devices connected in parallel, for which each electrochemical storage cell connected in series, and each set of electrochemical storage cells connected in parallel, is assigned a protective element which is arranged from outside and is electrically connected in parallel thereto. The purpose of the protective element in this arrangement is to allow any overvoltage which is applied to the terminals of the storage devices to be discharged via the protective element, so that the storage cell is not destroyed.
The storage devices themselves in their interior have a plurality of individual cells connected in parallel to one another, which consist at least of electrodes spaced with respect to one another and electrolyte arranged between the electrodes. As these individual cells also cannot be manufactured completely identically, they too, during charging, reach their maximum charge state at different times. To ensure against destruction of individual cells, the maximum charge voltage, particularly in the case of electrochemical storage devices containing gel electrolytes and solid electrolytes, depends on the weakest link. Consequently, all the components of these individual cells have to be adequately dimensioned for the desired charge voltage to be applied safely. This does entail an increase in mass, owing to which the actual mass of the electrochemical storage device exceeds the calculated required mass.
The object of the present invention is to provide an improved electrochemical storage device which makes it possible in a simple manner, especially with electrochemical storage devices of sandwich construction, to reliably avoid at least any destruction due to overvoltage, while keeping the total mass of the storage device as low as possible.
The object is achieved by the electrochemical storage device according to the invention in which a protective element whose electrical conductivity is voltage-dependent is arranged in direct contact with the two electrodes of each individual cell. In this manner, the electrochemical storage device is protected in a simple manner even when it is being assembled, and an external overcharge protection is no longer required. The resultant percentage increase in weight of the electrochemical storage device, if any, is small.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.